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[BUG]: geo projections lose their D3 default rotation

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
72/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
javascript
Domain
data-visualization

Research direction

Start at Geo.updateProjection and compare its rotation handling with the D3 projection constructors, especially the quincuncial and wiechel defaults described in the issue. Reproduce the peirce quincuncial example in the browser, then verify that the rendered orientation preserves the projection's canonical default while still applying a requested rotation.

Written by the indexing model from the issue text.

Description

bug P3 plotly-internal size: 3
Description

Geo.updateProjection calls projection.rotate([-rotation.lon, -rotation.lat, rotation.roll]) unconditionally. For a world-scope map projection.rotation defaults to [0, 0, 0], so any rotation the D3 projection ships with is discarded.

Six of the projections in plotly.js carry a non-identity default rotation:

projection D3 default rotation
albers [96, 0, 0]
bertin1953 [-16.5, -42, 0]
gringorten quincuncial [-90, -90, 45]
peirce quincuncial [-90, -90, 45]
sinu mollweide [-20, -55, 0]
wiechel [0, -90, 45]

For the quincuncials and wiechel that rotation is geometric: it places the sphere in the polar aspect and tilts the square 45°, and is applied inside the projection's own constructor:

// d3-geo-projection/src/quincuncial/index.js
return projection(projectQuincuncial)
    .rotate([-90, -90, 45])
    .clipAngle(180 - 1e-3);

Replacing it renders a different orientation from the one D3 defines. (For albers, D3's [96, 0, 0] preconfigures it for the USA, so discarding it is probably intended.)

Screenshots/Video

For 'peirce quincuncial':

D3 projection Plotly projection
Image Image
Steps to reproduce
  • Be on master
  • Open Plotly devtools
  • Enter the following snippet into the browser console:
Plotly.newPlot(gd, [{
    type: 'choropleth',
    locations: ['GHA', 'ARG', 'AUS', 'CAN'],
    z: [1, 2, 3, 4],
    showscale: false
}], {
    geo: { projection: { type: 'peirce quincuncial' }, fitbounds: false },
    height: 800
});
  • Note that
  • Expected: the canonical peirce quincuncial orientation.
  • Actual: rendered with rotation [0, 0, 0].
Screenshots
Notes
  • A fix needs to compose the requested rotation onto each projection's default rather than replace it, since d3.geoProjection.rotate() replaces
  • This was discovered in #7948. It's possible that this is intentional for a good reason, but it would be worth looking at changing it regardless.
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